An amount of water will separate from a cement slurry during the curing (or setting) process. This amount of water produced from the cement slurry is referred to as the free water. Historically, in order to determine the free water amount for a particular cement slurry, a standard American Petroleum Institute (API) test well known in the art has been utilized. During this API test, a 9.4 inch sample of the cement slurry is placed into a 1.5 inch diameter column. As the cement slurry sets, a small amount of water separates from the cement slurry and accumulates on the top of the column. After two hours, the amount of water is measured. This is the free water, and is expected in small amounts. An excessive amount is a symptom of poor cement and a poor cement slurry.
It is apparent that there are shortcomings with the API test. The free water measured is only determined after two hours; thus, this test is not informative as to how the free water disassociated from the cement slurry over time. While it is possible to run multiple samples, such a procedure would be lengthy and cumbersome. Moreover, this test for free water is performed at room temperature and at atmospheric pressure, rather than at the actual temperatures and pressures encountered in the well-bore environment. Since the rate at which a cement slurry cures is generally dependant upon temperature and pressure, the free water test is not performed under the conditions that will be encountered during actual use.
The advantages of testing cement and cement slurry over time under the well-bore conditions is recognized in the art. The standard device for testing cement at well-bore temperature and pressure is the UCA, which was originally introduced by Halliburton. The UCA was disclosed in U.S. Pat. No. 4,259,868 issued to Rao, et al. ("Rao") and teaches a method and apparatus for nondestructive testing of cement as a function of time and under temperature and pressure control. However, the UCA does not measure the free water of the cement slurry. The UCA design utilized two transducers in the pitch-catch mode. This mode of operation would allow the measurement of the average sound velocity of the material and attenuation of the material in the path of the acoustic signal. It could not, however, allow for the detection of layering in the material in the acoustic path. Since the free water forms as a layer above the cement slurry and cement, the design of the UCA could not measure the free water.
Therefore, there is a need in the art for an apparatus and method for measuring the free water as a function of time as it separates from a cement slurry and for measuring the free water as it separates from the cement slurry during curing under the temperature and pressure conditions the cement will encounter in the well-bore of an oil and gas well.